The state Supreme Court on Thursday ruled that one of the West Coast’s major players in the foreclosure industry violated the state consumer-protection law by falsely notarizing legal documents and not considering requests to delay the auction of a Whidbey Island home.

The court, overturning an appeals court’s decision, instructed a King County Superior Court judge to issue an injunction against foreclosure trustee Quality Loan Service.

Foreclosure trustees are not simply agents for the lender, the court wrote.

“The power to sell another person’s property, often the family home itself, is a tremendous power to vest in anyone’s hands,” Chambers wrote. The law “requires that trustee to be evenhanded to both sides and to strictly follow the law.”

In 2008, the nonprofit group Puget Sound Guardians sued Washington Mutual and Quality Loan for allegedly violating the consumer-protection law after the trustee sold Dorothy Halstien’s home at a foreclosure auction for a dollar more than the $83,087 the disabled senior owed, stripping her of more than $150,000 in equity.

The property’s new owners quickly flipped it, selling the property for $235,000.

Halstien owed WaMu about $75,000 at the time she developed dementia and had a guardian appointed.

The cost of her medical care ate up funds to pay the mortgage. She died in late 2008 at 76.

Quality falsely notarized the date on the notice of trustee sale, the court said, and apparently trained its notaries to do this regularly from 2004 to 2007. Had the notice of sale been correctly dated, the foreclosure auction would have been delayed at least a week, the court said.

“A signed notarization is the ultimate assurance upon which the whole world is entitled to rely that the proper person signed a document on the stated day and place,” Justice Chambers wrote.

Dianne Klem, executive director of Puget Sound Guardians, said her agency made repeated requests to WaMu and Quality to delay the sale. But Quality had a written agreement with WaMu that forbade it from postponing a sale without the bank’s approval.

An appeals court in 2011, rejecting a jury’s verdict against Quality, had ruled there wasn’t sufficient evidence to show Quality had violated the consumer-protection law — and as a result, was not liable for attorneys fees.

In a statement Thursday, Quality said it “has dedicated significant resources to ensure non-judicial foreclosures are processed to the highest standards of legal compliance while respecting and focusing on compassionate treatment of all borrowers during their difficult time.”

Fred Corbit, the senior attorney at the Northwest Justice Project who represented Puget Sound Guardians, said the court’s ruling sent a strong message to all foreclosure trustees:

“They have to treat both banks and borrowers in good faith, and if they don’t treat them in good faith then they’re going to be held liable for damages,” he said.